1. Field of the Invention
The present invention relates to a camera having a function of controlling the exposure of an image sensor so that a subject is photographed with appropriate brightness.
2. Description of the Prior Art
A camera is provided with an automatic exposure (AE) control function for controlling the exposure of an image sensor according to the brightness of a subject so that the subject, i.e. the main object to be photographed, is photographed with appropriate brightness. Such an AE function is provided not only in cameras such as video cameras and digital cameras that perform photoelectric conversion as an image sensor but also in cameras that use sensitized film as an image sensor.
In general, an AE function is achieved by determining the average brightness over a whole area that is going to be photographed and then adjusting the aperture diameter of an iris, or an aperture stop, so that the brightness so determined becomes equal to a predetermined value. This permits a subject to be photographed with appropriate brightness almost unfailingly in a front-lighted condition in which both the subject and the background are illuminated from the front (from the camera side) and there is little difference in brightness between the subject and the background. However, in a back-lighted condition in which the subject is illuminated from behind and the brightness of the subject is extremely low relative to the brightness of the background, the subject photographed appears dark. To avoid this, it is customary to determine the average brightness in different manners between in a front-lighted and a back-lighted condition so that a subject is photographed with appropriate brightness even against an extremely bright background. This treatment is called back-lighting compensation.
In conventional cameras provided with an AE function, a whole area to be photographed is divided into a plurality of regions, then brightness is determined in each of those regions, and then the aperture diameter of the iris is adjusted so that the average brightness over all those regions becomes equal to a predetermined target value. As the average brightness, an arithmetic average is used when back-lighting compensation is not performed, and a weighted average is used when back-lighting compensation is performed.
An example of weight factors used to determine a weighted average is shown in FIG. 6. In this example, the whole area to be photographed is divided into 64 regions, of which 24 regions R1 located in a central and a lower portion are assigned a weight factor of 1, 16 regions R2 located near the central portion are assigned a weight factor of 0.5, and the remaining 24 regions R3 located in an upper, a left-hand end, and a right-hand end portion are assigned a weight factor of 0. As long as a subject lies in the central and lower portions of the area to be photographed, the brightness of the background, lying in the upper, left-hand end, and right-hand end portions, is not reflected in the average brightness, and thus it is possible to photograph the subject with appropriate brightness.
In this way, in conventional cameras, back-lighting compensation is achieved by exploiting the fact that a subject is very likely to lie in a central or lower portion of an area to be photographed, and thus by using constant weight factors.
Examples of pictures taken with a conventional camera are shown in FIGS. 8A and 8B. FIG. 8A shows a picture taken without back-lighting compensation in a back-lighted condition. In this picture, the subject in a central portion appears dark. FIG. 8B shows a picture taken with back-lighting compensation in the same back-lighted condition. In this picture, the subject in a central portion appears appropriately bright.
However, a subject does not always lie in a central or lower portion of an area to be photographed, and this often makes it impossible to photograph the subject with appropriate brightness. FIG. 7 shows an example of brightness distribution that poses such a problem. FIG. 7 shows a subject (for example, two persons) lying in a left-hand end and a right-hand end portion of the area to be photographed in a back-lighted condition. Here, brightness is high in 28 regions RH, illustrated without hatching, located in a central and an upper portion, and brightness is increasingly low in the regions RM and RL, in this order, located in the left-hand end and right-hand end portions. Since the weight factor is great in the central portion, the high brightness of the background contributes greatly to the weighted average, and thus the subject appears dark in the picture taken. FIG. 8C shows an example of a picture in which the subject is not photographed with appropriate brightness even when back-lighting compensation is performed.
This problem can be overcome by distinguishing a front-lighted and a back-lighted condition on the basis of the difference in brightness observed within the area to be photographed and, if a back-lighted condition is recognized, adjusting the exposure of the image sensor so that the average brightness in low-brightness regions becomes equal to the target value. However, there may also occur an over-front-lighted condition in which a subject is illuminated from the front in such a way that the brightness of the subject is extremely high relative to the brightness of the background. Thus, simply distinguishing a front-lighted and a back-lighted condition does not permit a subject to be photographed with appropriate brightness in an over-front-lighted condition.